The known methods of removing carbon dioxide from gases include
(a) contacting the gas with a lime slurry, PA1 (b) contacting the gas with solid calcium hydroxide at elevated temperatures, typically 350.degree. C.-450.degree. C., and PA1 (c) contacting the gas with solid barium hydroxide hydrate at ambient temperatures. PA1 C is the carbon dioxide concentration upstream of the bed, PA1 T is the time required for the downstream concentration of carbon dioxide to reach 5% of the upstream concentration, and PA1 W is the weight of calcium hydroxide in the bed.
The reaction product calcium carbonate, or barium carbonate, resulting from these methods is highly stable and well suited for long term storage. This offers a very attractive chemical form for the fixation and disposal of radioactive carbon isotopes. However, each of the three methods has disadvantages. The lime slurry method has a contaminated liquid effluent; the solid calcium hydroxide system requires operation at elevated temperatures; while the barium hydroxide hydrate system has the serious disadvantage that barium hydroxide is both toxic and expensive. Applicants have tested a solid calcium hydroxide system at ambient temperatures (20.degree. C.-250.degree. C.), but this was found to be quite unsatisfactory as the conversion of Ca(OH).sub.2 to CaCO.sub.3 was found to be only about 3%.
However, the applicants have found, quite unexpectedly, that the solid calcium hydroxide system is most effective even at ambient temperatures if the humidity of the gas is raised to a value corresponding to a relative humidity of about 80% or higher measured at the bed temperature. Below 80% relative humidity there is absorption of the carbon dioxide, with consequent conversion of Ca(OH).sub.2 to CaCO.sub.3 as one would expect, and the conversion increases with relative humidity, but it is only when the relative humidity reaches about 80% that the conversion becomes high enough to be commercially useful. In fact, the conversion continues to increase rapidly throughout the 80%-100% relative humidity range. If the moisture content exceeds the upper limit of this range, however, the utilization of the Ca(OH).sub.2 is diminished.
Further investigation showed that the bed temperature may be as low as 10.degree. C. provided that the moisture content of the gas is suitably increased. In this case the relative humidity at the bed temperature should be between about 90%-100% for there to be useful conversion, i.e. utilization, of the calcium hydroxide. On the other hand, at higher bed temperatures the relative humidity of the gas may be considerably lower, and may be as low as about 40% if the temperature of the bed is 50.degree. C. The moisture content of the gas may be further reduced for higher bed temperatures, a useful conversion of the calcium hydroxide being obtained, but at bed temperatures above about 50.degree. C. one sacrifices the main advantage of the invention, namely the effective utilization of a calcium hydroxide bed operated at a coveniently low temperature. For the benefit of the invention to be fully realized the bed should be operated in the temperature range 10.degree. C.-50.degree. C., and preferably in the temperature range 20.degree. C.-30.degree. C., the moisture content of the gas stream being controlled accordingly.
U.S. Pat. No. 4,162,298 issued to David W. Holladay and Gary L. Haag, dated July 24, 1979, discloses a method for removing carbon dioxide from industrial off-gas using a particulate bed of barium hydroxide monohydrate wherein the gas is treated so that its relative humidity is in the range 10%-100%. However, no attempt was made to apply this method to a solid calcium hydroxide system, which was believed to be unsuitable. Moreover, subsequent work by Holladay and Haag, as reported in a presentation of the 16th DOE Nuclear Air Cleaning Conference, San Diego, Calif., Oct. 19-24, 1980, indicated that even the barium hydroxide system would be of little practical use if the relative humidity of the gas were too high, owing to degradation of the Ba(OH).sub.2 particles with resultant capillary condensation of water vapour and a consequent high pressure drop across the packed bed. The present applicants have discovered that the calcium hydroxide system does not have this drawback and will be most effective at very high relative humidities at which the barium hydroxide would be of little use.